1. Field of the Invention
The present invention relates to radio frequency (RF) power amplifiers.
2. State of the Art
In the area of RF communications, as the desire and need for greater transmission throughput grows, the preferred signals must exhibit high data efficiency. In general, high data efficiency signals exhibit the property of a varying envelope. Such envelope varying signals generally require linear circuit operation.
In a conventional radio transmitter, the modulator and power amplifier (PA) are separate functions, as shown in FIG. 1. The modulator translates the information input to the transmitter into a passband signal, usually at radio frequencies, which can have either a constant envelope (average signal power is equal to peak signal power) or a varying envelope (average signal power is less than peak signal power). It is generally understood in the radio art that circuitry that supports only constant envelope signals can be inherently more energy efficient than circuitry that supports envelope varying signals, since the envelope varying signal requires linear circuit performance. Since the envelope varying signal is more general, this disclosure is focused on the generation of these signals.
It is widely recognized that the desired joint goals of linear operation and high energy efficiency are mutually exclusive, that is, an amplifier with high energy efficiency does not operate in a linear manner, and similarly an amplifier operating in a linear manner does not exhibit high energy efficiency.
Since improving the energy efficiency of a linear amplifier (to 50% or higher) has historically proven to be essentially impossible, most efforts focus on taking a more efficient nonlinear amplifier and improving its linearity. These methods include feedforward linearization, predistortion, feedback predistortion, and modulator feedback. These methods are briefly summarized below.
In the feedforward linearization method shown in FIG. 2, the PA is first characterized as to what distortions to the signal it will generate. The inverse of these distortions is externally generated, and then summed with the output from the PA. This should cancel the distortions from the PA itself, resulting in improved linear operation.
In the predistortion method shown in FIG. 3, again the PA is first characterized as to what distortions to the signal it will generate. The inverse of these distortions is externally generated, and then applied to the input of the PA. The PA should cancel the distortions from the predistorter, resulting in improved linear operation.
In the feedback predistortion method shown in FIG. 4, again the PA is first characterized as to what distortions to the signal it will generate. The inverse of these distortions is externally generated, and then applied to the input of the PA. The PA should cancel the distortions from the predistorter, resulting in improved linear operation. Any errors in the predistortion are sensed at the output of the PA, and used to correct the predistorter. This technique is often called an adaptive predistorter.
Some efforts have begun recently to put continuous feedback around the PA back into the modulator, so that PA errors can be continuously corrected. Shown in FIG. 5, this is an extension of the adaptive predistorter method, where the predistorter function is included within the modulator. Because this is a feedback technique, stability of the feedback loop is of particular concern. Maintaining loop stability is further exacerbated by the inclusion of known nonlinear components (the PA) within the loop. Additionally, the allowable bandwidth of the signal modulation is restricted by the dynamics of the feedback loop. An example of this technique is the combined analog locked-loop universal modulator (CALLUM), described in D. J. Jennings, J. P. McGeehan, xe2x80x9cA high-efficiency RF transmitter using VCO derived synthesis: CALLUM,xe2x80x9d Proceedings of the 1998 IEEE Radio and Wireless Conference (RAWCON), August 1998, pp. 137-140.
The LINC technique (D. C. Cox, xe2x80x9cLinear Amplification with Nonlinear Components,xe2x80x9d IEEE Transactions on Communications, vol. COM-23, December 1974, pp. 1942-5) is an amplifier method that uses nonlinear amplifiers in combination to amplify an envelope-varying signal as shown in FIG. 6. The key is to represent an arbitrary bandpass (radio) signal as the vector sum of two constant envelope phase-modulated signals. The two constant envelope signals are amplified individually in high-efficiency nonlinear amplifiers which are each sized to provide one-half of the peak required output power, and then combined (usually in a passive network) for the final output signal. This combiner must internally dissipate the output power from both nonlinear PAs when the magnitude of the output signal is low. Thus, the LINC technique forfeits much of the inherent efficiency of the individual nonlinear amplifiers.
Another existing approach to amplifying envelope-varying signals with simultaneous high energy efficiency is envelope-elimination and restoration (EER), described in D. K. Su, W. J. McFarland, xe2x80x9cAn IC for Linearizing RF Power Amplifiers Using Envelope Elimination and Restoration,xe2x80x9d IEEE Journal of Solid-State Circuits, vol. 33, December 1998, pp. 2252-2258. The EER technique, like the LINC technique, is for a separate amplifier following the modulator as shown in FIG. 7. The EER amplifier must first demodulate the amplitude variations from the applied input signal, then limit the input signal for amplification in a nonlinear (preferably switch-mode) amplifier. Envelope restoration is achieved in the final output stage. A feedback loop is often used around the envelope restoration process to more closely match the output signal envelope with the measured envelope from the input signal. As with any feedback control loop, the loop dynamics restrict the achievable modulation bandwidth.
There remains a need to enable the generation of radio communication signals at a power suitable for transmission while simultaneously exhibiting high DC-to-RF conversion efficiency. Further, the quality of the signal so generated must meet often stringent performance specifications, such as the GSM-EDGE specifications.
The present invention, generally speaking, incorporates the power amplifier as a fundamental constituent of a modulator, using polar modulation techniques. Thus, it is possible to achieve the combination of precision signal generation (including envelope variations) along with high energy efficiency in combinations not possible heretofore. In accordance with one embodiment of the invention, a modulated radio (passband) signal generator produces high quality signals of general type, which specifically includes signals with varying envelopes. Signals are generated with high energy efficiency in the conversion of applied DC power to output RF signal power. The result is longer battery life for products such as mobile phone handsets. Dramatically improved efficiency also allows for a dramatic reduction (10 to 1 or greater) in the size of any required heatsink for the radio transmitter, which significantly lowers both cost and size. Furthermore, continuous operation of these radio transmitters is made possible with small temperature rises using small heatsinks, or even without any heatsink components. This provides for high operating reliability, as well as for greater throughput due to the longer operating time allowed. Another aspect of the invention allows the generation of high quality signals with wide bandwidth, without the need for continuous feedback during operation. This further reduces costs by greatly simplifying the design, manufacturing, and complexity of the transmitter circuitry.